Information technology systems, including storage systems, may need protection from site disasters or outages, where outages may be planned or unplanned. Furthermore, information technology systems may require features for data migration, data backup, or data duplication. Implementations for disaster or outage recovery, data migration, data backup, and data duplication may include mirroring or copying of data in storage systems. Such mirroring or copying of data may involve interactions among hosts, storage systems and connecting networking components of the information technology system.
A storage server, such as the IBM® TotalStorage® Enterprise Storage Server® (“ESS”), may be a disk storage server that includes one or more processors coupled to storage devices, including high capacity scalable storage devices, Redundant Array of Inexpensive (or Independent) Disks (“RAID”), etc. The enterprise storage servers are connected to a network and include features for copying data in storage systems.
Peer-to-Peer Remote Copy (“PPRC”) is an ESS function that allows the shadowing of application system data from a first site to a second site. The first site may be referred to as an application site, a local site, or a primary site. The second site may be referred to as a recovery site, a remote site or a secondary site. The logical volumes that hold the data in the ESS at the primary site are called primary volumes, and the corresponding volumes that hold the mirrored data at the secondary site are called secondary volumes. High speed links, such as IBM ESCON® links may connect the primary and secondary ESS systems.
In the synchronous type of operation for PPRC, i.e., synchronous PPRC, the updates done by a host application to the primary volumes at the primary site are synchronously shadowed onto the secondary volumes at the secondary site. As synchronous PPRC is a synchronous copying solution, write updates are ensured on both copies (primary and secondary) before the write is considered to be completed for the host application. In synchronous PPRC the host application does not get the “write complete” response until the update is synchronously done in both the primary and the secondary volumes. Therefore, from the perspective of the host application, the data at the secondary volumes at the secondary site is equivalent to the data at the primary volumes at the primary site.
Inherent to synchronous PPRC operations is an increase in the response time as compared to asynchronous copy operation. The overhead comes from the additional steps which are executed before the write operation is signaled as completed to the host application. Also, the PPRC activity between the primary site and the secondary site may be comprised of signals and data which travel through the links that connect the sites, and the overhead response time of the host application write operations will increase proportionally with the distance between the sites. Therefore, the distance affects a host application's response time. In certain implementations, there may be a maximum supported distance for synchronous PPRC operations referred to as the synchronous communication distance.
In the Extended Distance PPRC method of operation, PPRC mirrors the updates of the primary volume onto the secondary volumes in an asynchronous manner, while the host application is running. In asynchronous PPRC, the host application receives a write complete response before the update is copied from the primary volumes to the secondary volumes and a host application's write operations are free of the typical synchronous overheads. Therefore, asynchronous PPRC is suitable for secondary copy solutions at very long distances with minimal impact on host applications. However, asynchronous PPRC does not continuously maintain a consistent (point-in-time) copy of the primary data at the secondary site, therefore risking data loss in certain circumstances.